How many watts is 1 horsepower?

One mechanical (imperial) horsepower equals exactly 745.69987158227022 watts, commonly rounded to 745.7 W. A car engine's 300 hp rating, for example, equals about 223,700 watts (223.7 kW).

How do you convert horsepower to kilowatts?

Multiply horsepower by 0.7457 (the exact factor is 0.74569987158227022) to get kilowatts. 200 hp is approximately 149.1 kW.

Why is horsepower defined as such an odd number of watts?

Horsepower predates the watt by roughly a century and was originally defined as 550 foot-pounds-force per second — a figure attributed to James Watt's estimate of a draft horse's sustained work rate. When the watt (itself named after Watt) was later formally defined in SI terms, the two units' relationship simply fell out as 745.7 W, an artifact of one unit being retrofitted onto the other rather than designed to align with it.

What's a typical horsepower output for a human?

A standard human can sustain roughly 0.1 horsepower (about 75 W) indefinitely during continuous physical exertion — a useful reference point for how much power modern engines and motors produce relative to unassisted human effort.

Guide

Watts vs horsepower: converting car and motor power

Mechanical (imperial) horsepower is defined as exactly 550 foot-pounds-force per second, which works out to 745.69987158227022 watts — an odd-looking number with a precise historical origin.

By Buğra SözeriPublished September 25, 2026

Watts and horsepower both measure power — the rate energy is transferred or work is done — but they come from entirely different eras and traditions. The watt is the clean SI derived unit, one joule per second. Horsepower predates it by roughly a century, originally devised to compare the output of early steam engines against the animal labor they were replacing.

The exact conversion

Mechanical (imperial) horsepower is defined as exactly 550 foot-pounds-force per second, which converts to SI units as:

1 hp = 745.69987158227022 W
1 W ≈ 0.00134102 hp

Reference values

Horsepower	Watts	Example
0.1 hp	≈ 75 W	Sustained human power output
1 hp	745.7 W	Reference unit
150 hp	≈ 111,855 W (111.9 kW)	Compact car, low end
300 hp	≈ 223,710 W (223.7 kW)	Mid-size performance car

Why watts are taking over car specs

Electric vehicle motors are almost universally rated in kilowatts rather than horsepower, since watts are the native SI unit for electrical power and EV manufacturers, particularly outside the US, market performance in kW directly. US-market listings often still convert that kW figure to horsepower for consumer familiarity, which means the same EV motor can appear with two different-looking numbers depending on the region a spec sheet was written for — both describing the identical power output.

Frequently asked questions

How many watts is 1 horsepower?: One mechanical (imperial) horsepower equals exactly 745.69987158227022 watts, commonly rounded to 745.7 W. A car engine's 300 hp rating, for example, equals about 223,700 watts (223.7 kW).
How do you convert horsepower to kilowatts?: Multiply horsepower by 0.7457 (the exact factor is 0.74569987158227022) to get kilowatts. 200 hp is approximately 149.1 kW.
Why is horsepower defined as such an odd number of watts?: Horsepower predates the watt by roughly a century and was originally defined as 550 foot-pounds-force per second — a figure attributed to James Watt's estimate of a draft horse's sustained work rate. When the watt (itself named after Watt) was later formally defined in SI terms, the two units' relationship simply fell out as 745.7 W, an artifact of one unit being retrofitted onto the other rather than designed to align with it.
What's a typical horsepower output for a human?: A standard human can sustain roughly 0.1 horsepower (about 75 W) indefinitely during continuous physical exertion — a useful reference point for how much power modern engines and motors produce relative to unassisted human effort.

Sources & references

Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.

BIPM — SI Brochure, 9th edition (watt defined as SI derived unit of power) — Formal SI definition of the watt as one joule per second(as of 2026-09-25)